The Pediatric Airway

Tongue

It is generally held that the tongue of the neonate and infant is relatively large in proportion to the rest of the oral cavity and therefore more easily obstructs the airway, especially in the neonate. However, magnetic resonance imaging (MRI) studies demonstrated that in children 1 to 11 years of age, there is proportional growth of the tongue and other soft tissues in relation to the bony structures of the oral cavity although that study did not examine neonates and infants (who are 1-12 months of age). The contribution of the tongue to upper airway obstruction with sedation or induction of anesthesia is relatively minor; much of the obstruction in older children is more likely attributable to the collapse of the nasopharynx and epiglottis, although the tongue may still contribute to obstruction in all age groups.

Position of the Larynx

The larynx in the infant is more cephalad than in the adult, classically described at the level of C3-4 in the former compared with C4-5 in the latter ( Fig. 14.1 ). MRI and computed tomography (CT) have confirmed the more cephalad position of the larynx in children and demonstrated that the hyoid bone is at the level of C2-3 in infants and children up to 2 years of age. Consequently, the distances between the tongue, hyoid bone, epiglottis, and roof of the mouth in infants are less than those in older children or adults.

The proximity of the tongue base to the more superior larynx also makes visualization of laryngeal structures more difficult because it produces a more acute angle between the plane of the tongue and the plane of the glottic opening. It is for this reason that a straight laryngoscope blade, which lifts the tongue from the field of view during laryngoscopy, facilitates visualization of an infant's larynx. This anatomic relationship is further complicated in certain conditions such as the Treacher Collins anomaly and other syndromes associated with mandibular and midfacial hypoplasia that make direct visualization of the glottis difficult and sometimes impossible with direct laryngoscopy ( Fig. 14.2 ). The reason for this difficulty is that with mandibular and midfacial hypoplasia, the base of the tongue is positioned more caudally (known as glossoptosis), in closer proximity to the laryngeal inlet than normal. The result is a more acute angle between the plane of the tongue and the plane of the laryngeal inlet (often 90 degrees) ( Fig. 14.3 ). In this situation, conventional rigid laryngoscopy provides excellent visualization of the esophageal inlet rather than the laryngeal inlet, necessitating the use of special equipment or special techniques to intubate the trachea.

Epiglottis

The epiglottis in the infant is narrow, omega shaped, and angled away from the axis of the trachea, which contrasts with that in the adult, which is flat and broad, and its axis is parallel to the trachea ( Figs. 14.4 and 14.5 ). This shape allows the epiglottis to approach the uvula during infant breastfeeding; separating breath from fluid and allowing respiration at the same time as swallowing. The shape of epiglottis makes it more difficult to directly lift in the neonate and infant with the tip of a laryngoscope blade.

Vocal Folds

The vocal folds (cords) in an infant are angled such that the anterior insertion is more caudad than the posterior insertion, whereas the axis of the folds in the adult is perpendicular to that of the trachea (compare Fig. 14.4A with Fig. 14.5A ). This anatomic feature alters the angle at which the tracheal tube approaches the laryngeal inlet and occasionally leads to difficulty with tracheal intubation, especially with the nasal approach. In the latter case, the tip of the endotracheal tube (ETT) may be held up at the anterior commissure of the vocal folds.

Subglottis

Classic teaching holds that the narrowest part of an infant's larynx is the cricoid cartilage; in an adult, it is the rima glottidis. This teaching was supported by an MRI and CT studies in young children (<2 years of age) who were sedated with oral medications and breathing spontaneously. In contrast, another study in children 2 months to 13 years of age undergoing MRI with propofol sedation and spontaneous respirations reported that the narrowest portions of the pediatric larynx were the glottic opening and the immediate sub–vocal cord level and that this finding did not change relative to the dimensions of the cricoid ring throughout childhood. These observations contradict the classic observations of autopsy specimens dating back to 1897 and other subsequent anatomic autopsy studies. The most likely reason for these apparent contradictory observations is that the more recent studies were conducted in spontaneously breathing children with variable portions of the respiratory cycle and that soft tissue collapse gives the appearance of a narrower airway above the cricoid cartilage. Nonetheless, when a relatively large-diameter tube is inserted into the glottic aperture, the tube readily passes through the distensible vocal cords but may meet resistance below the cords (e.g., the nondistensible cricoid ring region). Although these studies demonstrate in vivo dynamic, physiologic relationships, the cricoid cartilage is functionally the narrowest portion of the upper airway.

Growth of the subglottic airway occurs rapidly during the first 2 years of life; thereafter, growth of the airway is linear. The cricoid and thyroid cartilages reach adult proportions by 10 to 12 years of age, thus eliminating both the angulation of the vocal cords and the narrow subglottic area.

In adults, the rima glottidis is the narrowest part of the airway, and an ETT that traverses the glottis passes into the trachea without resistance. However, in about 70% of adult cadavers, the narrowest portion of the airway was identified in the subglottic region. The range in diameter for adult females is 10 to 16 mm, and for adult males it is 13 to 19 mm. The likely reason that ETTs pass easily through the rima glottidis into the trachea of an adult is that overall, the narrowest portion of the airway is still larger than the most commonly used ETT sizes. The apparent subglottic narrowing in adults is generally not evident unless there is the need to pass a larger-diameter ETT such as a double-lumen tube. In contrast, in a child, it is common for an ETT to pass easily through the vocal folds (glottic opening) but not through the subglottic region ( Fig. 14.6 ; see Video 14.1 ). The larynx in both children and adults should be considered funnel-shaped, although this configuration is exaggerated and is of greater importance in infants and young children than in adults.

The cricoid is the only complete ring of cartilage in the laryngotracheobronchial tree; as such it is nondistensible. Because the mucosa that lines the upper airway is loose-fitting pseudostratified columnar epithelium, pressure on the mucosa may cause reactive edema that encroaches on the diameter of the lumen. A tight-fitting ETT that compresses the tracheal mucosa at this level may cause inflammation and edema when it is removed, reducing the luminal diameter and increasing the airway resistance at the time of extubation (e.g., postextubation croup). Because the subglottic region in the infant is smaller than in the adult, the same degree of airway edema results in greater resistance in the infant. For example, assuming that the diameter of the cricoid ring in the infant is 4 mm and the diameter of the adult cricoid ring or trachea is 8 mm, 1 mm of edema circumferentially within the airway (i.e., reduction of the diameter of the airway by 2 mm) would decrease the cross-sectional area of the airway in the infant by approximately 75% (to 2 mm), whereas the area in the adult would decrease by only 44% (to 6 mm). Physiologically, because the resistance to airflow in the upper airway is turbulent, this reduction in diameter of the upper airway would increase the resistance to flow by the radius to the fifth power , or 32-fold, in the infant, compared with 5-fold in the adult. ( Fig. 14.7 ).

Anatomy

Structure

The larynx is composed of 1 bone (hyoid) and 11 cartilages (the single thyroid, cricoid, and epiglottic cartilages and the paired arytenoid, corniculate, cuneiform, and triticeal cartilages). These cartilages are suspended by ligaments from the base of the skull. The body of the cricoid cartilage articulates posteriorly with the inferior cornu of the thyroid cartilage. The paired triangular arytenoid cartilages rest on top of, and articulate with the superoposterior aspect of the cricoid cartilage. The arytenoid cartilages are protected by the thyroid cartilage ( Fig. 14.8 ). The triticeal cartilages are rounded nodules of cartilage, approximately the size of a pea in adults, located in the margins of the lateral thyrohyoid ligament.

Tissue folds and muscles cover these cartilages. In contrast to adults, but comparable to most mammals, the cartilaginous glottis accounts for 60% to 75% of the length of the vocal folds in children younger than 2 years of age. Contraction of the intrinsic laryngeal muscles alters the position and configuration of these tissue folds, thus influencing laryngeal function during respiration, forced voluntary glottic closure (Valsalva maneuver), reflex laryngospasm, swallowing, and phonation ( Fig. 14.9 ).

The laryngeal tissue folds consist of the following:

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  Paired aryepiglottic folds extending from the epiglottis posteriorly to the superior surface of the arytenoids (the paired cuneiform and corniculate cartilages lie within for support and reinforcement, much like the metal stays in a shirt collar)

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  Paired vestibular folds (false vocal cords) extending from the thyroid cartilage posteriorly to the superior surface of the arytenoids

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  Paired vocal folds (true vocal cords) extending from the posterior surface of the thyroid plate to the anterior projection or vocal process of the arytenoids

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  A single interarytenoid fold (composed of the interarytenoid muscle covered by tissue) bridging the arytenoid cartilages

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  A single thyrohyoid fold extending from the hyoid bone to the thyroid cartilage

Function

Obligate Nasal Breathing

Infants are considered obligate nasal breathers. Obstruction of their anterior or posterior nares (nasal congestion, stenosis, choanal atresia) can cause asphyxia. Immaturity of coordination between respiratory efforts and oropharyngeal motor and sensory input accounts in part for obligate nasal breathing. Furthermore, because the larynx is more cephalad in the neck of an infant and oropharyngeal structures are closer together, the tongue rests against the roof of the mouth during quiet respiration, resulting in oral airway obstruction. Multiple sites of pharyngeal airway obstruction may also contribute to airway obstruction when the infant attempts to breathe against a partially obstructed upper airway or with relaxation of upper airway muscle tone after sedation or induction of anesthesia.

The ability to coordinate breathing and swallowing improves as the infant matures. The larynx enlarges and moves more caudad in the neck as the cervical spine lengthens and the infant begins to breathe adequately through the mouth. This matures by age 3 to 5 months. The ability to breathe through the mouth when the nares are obstructed is age dependent: 8% of preterm infants of 31 to 32 weeks postconception age were able to breathe through the mouth in response to nasal occlusion compared with 28% of more mature preterm infants of 35 to 36 weeks postconception age ; approximately 40% of full-term infants can switch from nasal to oral breathing. However, the ability of premature neonates to breathe through the mouth may not be as poor as these early reports suggested. Slow and fast nasal occlusion applied to 17 healthy preterm infants (gestational age, 32 ± 1 weeks; postnatal age, 12 ± 2 days) led to a switch from nasal to oral breathing. These improved results were attributed to the more extended observation period (>15 seconds) in the later study. The presence of a nasogastric tube may also affect the infant's breathing if the “unobstructed” nasal passage has an existing underlying obstruction.

Tracheal and Bronchial Function

Tracheal and bronchial diameters are a function of elasticity and of distending or compressive forces ( Fig. 14.10 ). The larynx, trachea, and bronchi in the infant are quite compliant compared with those in the adult and therefore are more subject to distention and compression forces. The intrathoracic trachea is subject to stresses that are different from those in the extrathoracic portion. During expiration, intrathoracic pressure remains slightly negative, maintaining patency of the intrathoracic trachea and bronchi (see Fig. 14.10B ). During inspiration, a greater negative intrathoracic pressure dilates and stretches the intrathoracic trachea and bronchi. The extrathoracic trachea at the thoracic inlet is slightly narrowed by dynamic compression that results from the differential between intratracheal and atmospheric pressures. However, the cartilages of the trachea, along with the muscles and soft tissues of the neck, maintain patency of the airway (see Fig. 14.10A ).

Obstruction of the extrathoracic upper airway that can occur with epiglottitis, laryngotracheobronchitis, or an extrathoracic foreign body alters normal airway dynamics. Inspiration against an obstruction results in more negative intrathoracic pressure, further dilating the intrathoracic airways. Clinically, the net effect is a dynamic collapse of the extrathoracic trachea below the level of the obstruction. This collapse is maximal at the thoracic inlet, where the greatest pressure gradient exists between negative intratracheal and atmospheric pressures. As a result, inspiratory stridor is prominent (see Fig. 14.10C and Video 14.1 ). With intrathoracic tracheal obstruction (e.g., foreign body, vascular ring) (see Video 14.1 ), stridor may occur during both inspiration and expiration. In lower airway obstruction (e.g., asthma, bronchiolitis), significant intrathoracic tracheal and bronchial collapse may occur as a result of the prolonged expiratory phase and greatly increased positive extraluminal pressure (see Fig. 14.10D ). In addition, because the airways in children are very compliant, they may be more susceptible to closure during bronchial smooth muscle contraction (e.g., with reactive airway disease). Preterm and term infants may experience airway closure even during quiet respiration.

Avoiding dynamic airway collapse is particularly important. The very compliant trachea and bronchi of an infant or child are prone to collapse, particularly at the extremes of transluminal pressures that may occur when a child is crying vigorously. The susceptibility of a child to these dynamic forces on the airway is inversely related to age, with preterm infants being most susceptible and adults being least susceptible. For this reason, it is essential that children with airway obstruction remain calm. Skill and understanding are required on the parts of the parents, nursing staff, and physicians. Sedatives and opioids should be used with caution before insertion of an ETT, because they may depress or ablate the life-sustaining voluntary efforts to breathe, resulting in significant morbidity or mortality.

Work of Breathing

Work of breathing (WOB) may be defined as the product of pressure and volume. It may be analyzed by plotting transpulmonary pressure against tidal volume. The WOB per kilogram body weight is similar in infants and adults. However, the oxygen consumption of a full-term neonate (5–7 mL/kg per minute) is several times that of an adult (2-3 mL/kg per minute). This greater oxygen consumption (and greater carbon dioxide production) in infants accounts in part for their increased respiratory frequency compared with older children. In preterm infants, the oxygen consumption related to breathing is three times that in adults.

The location of airway resistance within the tracheobronchial tree differs between infants and adults. The nasal passages account for 25% of the total resistance to airflow in a neonate, compared with 60% in an adult. In infants, most resistance to airflow occurs in the bronchi and small airways. This results from the relatively smaller diameter of the airways and the greater compliance of the supporting structures of the trachea and bronchi. In particular, the soft cartilaginous chest wall of a neonate is very compliant; the ribs provide less support to maintain negative intrathoracic pressure. This lack of negative intrathoracic pressure combined with the increased compliance of the bronchi can lead to functional airway closure with every breath. In infants and children, therefore, small-airway resistance accounts for most of the WOB, whereas in adults, the nasal passages provide the major proportion of flow resistance.

In the presence of increased airway resistance or decreased lung compliance, an increased transpulmonary pressure is required to produce a given tidal volume, and therefore the WOB is increased. Any change in the airway that increases the WOB may lead to respiratory failure. Recall that the WOB (resistance to air flow) is inversely proportional to the fourth power of the radius of the lumen during laminar flow (beyond the fifth bronchial division) and to the fifth power of the radius during turbulent flow (upper airway to the fifth bronchial division). Because the diameter of the airways in infants is smaller than in adults, pathologic narrowing of the airways in infants exerts a greater adverse effect on the WOB. Increase in the WOB may also occur with a long ETT of small diameter, an obstructed ETT, or a narrowed airway. All of these situations increase oxygen consumption, which in turn increases oxygen demand. The increased oxygen demand is initially addressed by an increase in respiratory rate, but the increased WOB may not be sustainable. The end result may be exhaustion, which leads to respiratory failure (CO ₂ retention and hypoxemia) ( Fig. 4.9 ).

The difference in histology of the diaphragm and intercostal muscles of preterm and full-term infants compared with older children contributes to increased susceptibility of infants to respiratory fatigue or failure. Type I muscle fibers permit prolonged repetitive movement; for example, long-distance runners through repeated exercise increase the proportion of type I muscle fibers in their legs. The percentage of type I muscle fibers in the diaphragm and intercostal muscles increases with age (preterm infants < full-term infants < 2-year-old children) ( Fig. 14.11 ). Any condition that increases the WOB in preterm and full-term neonates may fatigue the respiratory muscles and precipitate respiratory failure more readily than in an adult.

Airway Obstruction During Anesthesia

Airway obstruction during anesthesia or loss of consciousness appears to be most frequently related to loss of muscle tone in the pharyngeal and laryngeal structures rather than apposition of the tongue to the posterior pharyngeal wall. The progressive loss of tone with deepening anesthesia results in progressive airway obstruction primarily at the level of the soft palate and the epiglottis. In children, the pharyngeal airway space decreases in a dose-dependent manner with increasing concentrations of both sevoflurane and propofol anesthesia. This reduction in pharyngeal space has been observed mainly in the anteroposterior dimension. As the depth of propofol anesthesia in children increases, upper airway narrowing occurs throughout the entire upper airway but is most pronounced in the hypopharynx at the level of the epiglottis. Extension of the head at the atlantooccipital joint with anterior displacement of the cervical spine (sniffing position) improves hypopharyngeal airway patency but does not necessarily change the position of the tongue. This observation supports the concept that upper airway obstruction is not primarily caused by changes in tongue position but rather by collapse of the pharyngeal structures.

Pharyngeal airway obstruction also occurs during obstructive sleep apnea in children and adults. The sniffing position increases the cross-sectional area and decreases the closing pressure of both the retropalatal and the retroglossal space in anesthetized adults with obstructive sleep apnea. The application of continuous positive airway pressure (CPAP) is a common method to overcome such airway obstruction (see Figs. 33.10 and 33.11 ). During propofol anesthesia in children, CPAP works primarily by increasing the transverse dimension of the airway. This occurs despite the fact that anesthesia obstructs the airway mostly by narrowing the anteroposterior dimension. Chin lift and jaw thrust also improve airway patency in anesthetized children with adenotonsillar hypertrophy. Lateral positioning (also known as the “recovery or tonsillectomy position”) dramatically enhances the effects of these airway maneuvers ; lateral positioning alone improves airway dimensions. Compared with chin lift and CPAP, the jaw thrust maneuver is the most effective means to improve airway patency and ventilation in children undergoing adenotonsillectomy (see Video 14.2 ).

Clinical Evaluation

Diagnostic Testing

Routine evaluation of the airway usually requires only a careful history and physical examination. In the presence of airway pathology, however, laboratory and radiologic evaluation can be extremely valuable. Radiographs of the upper airway (anteroposterior and lateral films and fluoroscopy) may provide evidence about the site and cause of airway obstruction. When necessary, MRI, CT, and three-dimensional (3-D) modeling provide more detailed information. Radiologic airway examination in a child with a compromised airway may be undertaken only if there is no immediate threat to the child's safety and only in the presence of skilled and appropriately equipped personnel able to manage the airway. Securing the airway through tracheal intubation must not be postponed to obtain a radiologic diagnosis when the child has severely compromised air exchange. Blood gas analysis is occasionally of value for assessing the degree of physiologic compromise, especially with chronic airway obstruction and compensated respiratory acidosis. Performing an arterial (or venous) puncture for blood gas analysis may provide helpful information, but it is often upsetting to the child and may risk aggravation of the underlying airway obstruction through dynamic airway collapse. Candidates for blood gas analysis must be carefully selected and the procedure skillfully performed.

Endoscopic evaluation (flexible fiberoptic endoscopy) of the airway before tracheal intubation can be useful in infants and in cooperative older children if a glottic pathologic process is suspected or if difficulty is anticipated when visualizing the glottis. Ultrasound can also be used to examine the airway. It can be used to determine the internal diameter of the trachea, which can help with the selection of the optimal ETT size.